Silent carriers may have only mild purulent debris in the guttural pouches. Chondroid formation is not required to establish
the silent-carrier state.
Farms with recurrent disease can harbor asymptomatic carriers that make up 16 percent to as much as 69 percent of the general
population, Holland says.
"The old myths about farms that have strangles being stuck with it for a long period have recently been proven wrong," Holland
adds. "Research shows that horses can carry S. equi in their guttural pouches for years and show no clinical signs. Farms that have had recurrent problems most likely have
asymptomatic carriers in their herd."
"When you look at the environment — the fields, the farm structures, the wood, the troughs — that's rarely the source of the
continuation of the disease unless there's an affected animal out there," Sweeney explains.
"Eventually, over a matter of weeks, the fields, the stall walls, the water buckets, etc., are fine, but as long as there's
that one horse that is not completely disease-free, although it might look clinically recovered, it is asymptomatic and available
to continue to spread disease to other previously unaffected horses," Sweeney says.
"Persistence of strangles on a farm has long been blamed on contamination of the soil with S. equi," says Phoebe Smith, DVM, Dipl. ACVIM, assistant professor of equine internal medicine in the Department of Veterinary Clinical
Sciences at The Ohio State University College of Veterinary Medicine.
"This is rarely the case, except when pus or contaminated discharge freeze before natural soil bacteria kill the Strep equi," Smith explains.
Only these frozen bacteria can persist in the environment to infect horses when the ground thaws. Strangles bacteria can survive
three to four weeks in water troughs.
"In the absence of clinical disease, persistence of S. equi may be due to asymptomatic or 'silent' carriers," says Smith.
The most common site of bacterial persistence, or carriage, is the guttural pouches; carriers may intermittently shed the
bacteria for months to years. That scenario can result in endemic infections on farms, causing frequent outbreaks and substantial
Chondroids (hard bodies, formed of dry exudates) may remain in the guttural pouches for years and be a source of transmission
in otherwise healthy-looking animals.
Methods of control
Control of strangles traditionally was based on either a four-week quarantine without bacteriological screening, or on the
collection of three consecutive negative nasopharyngeal swabs.
But carriers that shed S. equi intermittently still might culture negatively for weeks or months.
The current gold standard that can detect even these carriers is culture of lavage samples collected endoscopically from the
Another current method of carrier detection is the PCR (polymerase chain reaction) designed to detect DNA from the gene of
S. equi. According to Sweeney, this assay may be used in conjunction with guttural-pouch endoscopy and is useful to detect asymptomatic
carriers, determine infection status prior to or after transportation to another facility or prior to commingling with healthy
animals or determine the success of elimination of S. equi from the guttural pouch.
Smith adds, Screening for silent carriers of Strep equi is useful in varying scenarios, including:
- the endemic farm with an unknown source of infection,
- new horses being shipped to strangles-free farms,
- horses recovered from clinical disease prior to competing, training or otherwise commingling.
A thorough look at methodology for screening horses for silent carriers of S. equi was initiated on three farms, and the results presented at the 2006 convention of the American Association of Equine Practitioners